Conventionally, as an example of an optical fiber structure, an optical element combination structure (an optical module) in which a V-shaped cross-sectional groove and an optical waveguide are integrally formed and an optical fiber disposed on the V-shaped cross-sectional groove and the optical waveguide are coupled to each other has been known (Please refer to, for example, Patent Publications 1-3 below). Now, such an optical element combination structure will be explained by referring to optical element combination structures disclosed in the Patent Publications 1 and 3 as examples.
FIG. 7 shows an optical element combination structure disclosed in the Patent Publication 1. An optical element combination structure 70 has an optical fiber 72 and a substrate 76 formed with an optical waveguide 74 to be aligned with the fiber 72. The substrate 76 has a V-shaped cross-sectional groove 78 formed so that, when the fiber 72 is disposed onto the V-shaped cross-sectional groove 78, the fiber 72 and the waveguide 74 are aligned with each other, and a recess 80 formed on a waveguide side relative to the V-shaped cross-sectional groove 78.
The fiber 72 is disposed onto the V-shaped cross-sectional groove 78 so that a tip of the fiber 72 protrudes into the recess 80, and the tip is abutted to an input of the waveguide 74. This allows the fiber 72 and the waveguide 74 to be aligned with each other, namely, enables them to be coaxially centered. Then, the fiber 72 is fixed to the V-shaped cross-sectional groove 78 with an adhesive. Thus, the alignment between the fiber 72 and the waveguide 74 can be maintained.
FIG. 8 shows an optical element combination structure disclosed in the Patent Publication 3. An optical element combination structure 90 has an optical fiber 91, a substrate 93 formed with an optical waveguide 92 to be aligned with the fiber 91, a fixing groove 94 for fixing the fiber 91 thereon, and an adhesive-separating groove 95 formed across the fixing groove 94. Between the fiber 91 and the waveguide 92, a small mount of an ultraviolet curing type adhesive 96 for connecting end surfaces thereof is dropped. Between the fiber 91 and the substrate 93, a fixing adhesive 97 is applied.
In a state that the fiber 91 and the waveguide 92 are coaxially centered, the ultraviolet curing type adhesive 96 dropped between the fiber 91 and the waveguide 92 is cured so that the fiber 91 and the waveguide 92 are firmly adhered to each other. Then, the fixing adhesive 97 is cured so that the fiber 91 and the substrate 93 are adhered to each other. Since the end-surface-connecting adhesive 96 and the fixing adhesive 97 are separated from each other by an adhesive-separating groove 95, even though the fixing adhesive 97 is shrunk when it is cured, the end-surface-connecting adhesive 96 is prevented from being withdrawn by the fixing adhesive 97 so that the alignment between the fiber 91 and the waveguide 92 can be prevented from shifting. A value of coupling loss of the optical element combination structure 90 at a room temperature of 25° C. is maintained equal to or less than 0.5 dB.
Further, conventionally, as another example of an optical fiber structure, an optical fiber structure in which a fiber is positioned by a V-shaped cross-sectional groove formed on a substrate and the fiber is fixed between the substrate and the holding member with an adhesive has been known. Such an optical fiber structure is known, for example, as an optical fiber array; an optical element combination structure (an optical module) in which a V-shaped cross-sectional groove and an optical waveguide are integrally formed and the optical waveguide and an optical fiber positioned on the V-shaped cross-sectional groove are coupled to each other; and an optical element combination structure (an optical module) in which an optical fiber array and an optical waveguide are coupled to each other (Please refer to, for example, Patent Publication 4 below).
FIG. 26 is a partially cross-sectional front view of an example of an optical element combination structure (an optical module) in which optical fibers positioned on respective V-shaped cross-sectional grooves and an optical waveguide are coupled to each other. FIG. 27 is a cross-sectional view taken along the line XXVII-XXVII in FIG. 26. An optical element combination structure 200 has upstream optical fibers 202 each having an end surface 202a and extending in a longitudinal direction; a downstream optical fiber 204 having an end surface 204a opposed to the end surfaces 202a of the upstream fibers 202 and extending in the longitudinal direction; and an optical waveguide 206 disposed between the upstream fibers 202 and the downstream fiber 204 so that a light is transmitted from the former to the latter. The combination structure 200 further has a substrate 210 provided with V-shaped cross-sectional grooves for receiving and positioning the upstream fibers 202 and the downstream fiber 204; holding blocks 212, 214 respectively covering the upstream fibers 202 and the downstream fiber 204 from their upper sides and pushing them against the substrate 210; and an adhesive 216 with which spaces formed between any two of the substrate 210, the fibers 204, 206 and the holding blocks 212, 214 are filled to fix them to each other. The holding blocks 212, 214 respectively have contact surfaces 218 contacting the upstream fibers 202 and the downstream fiber 204.
In this combination structure 200, lights passing through the upstream fibers 202 are transmitted via the waveguide 206 to the downstream fiber 204.
Patent Publication 1: Japanese Patent Laid-open Publication No. 1-126608 (FIG. 1)
Patent Publication 2: Japanese Patent Laid-open Publication No. 2001-281479 (Paragraph 0017 and FIG. 1)
Patent Publication 3: Japanese Patent Laid-open Publication No. 2000-105324 (claim 1, Paragraph 0052 and FIG. 1b)
Patent Publication 4: Japanese Patent Laid-open Publication No. 2003-322744 (FIGS. 1-5)